It's a tough decision. Both systems have so few good games to offer.. You can either get lame FPSes or... just crap on the Wii. But you spend less on media for the Wii, so I'd go with that.
No wait. Wasn't there some issue with the Xbox 360.. I swear something was...funny with the console.. Oh yeah, it breaks. There's where you stop thinking about a 360. You learn from the mistakes of anyone who's owned one, and don't buy/trade one.
My theory is that the console has a 100% failure rate, because everyone I know (save Earthbound from these forums, but his will break too, he just never, ever, ever plays it) has had their 360 RRoD on them. The hardware is flawed. You can't have a ridiculously powerful graphics processor cooled by a 3mm heat sink, especially when that heat sink runs flush into a DVD-rom drive.